Alkali-Promoted Activated Alumina Adsorbent

ABSTRACT

An adsorbent for removing CO 2  from a gas mixture, the adsorbent comprising alumina and a carbonate compound where the carbonate to alumina IR absorbance intensity ratio is reduced by washing the adsorbent with water. The disclosure also describes a method of making adsorbent particles, process for removing CO 2  from a gas mixture using the adsorbent, and an adsorption unit using the adsorbent.

BACKGROUND

Before air can be introduced into a cryogenic air separation process inwhich oxygen and nitrogen are separated from one another, it isnecessary to remove carbon dioxide present in the air at low levels,e.g. 400 ppm. If this is not done, the carbon dioxide will solidify inthe air separation plant. Two methods generally used for such carbondioxide removal are pressure swing adsorption (PSA) and temperatureswing adsorption (TSA).

In each of these techniques, a bed of adsorbent is exposed to a flow offeed air for a period of time to adsorb CO₂ from the air. Thereafter,the flow of feed air is shut off from the adsorbent bed and theadsorbent is exposed to a flow of purge gas which strips the adsorbedCO₂ from the adsorbent and regenerates the adsorbent for further use. InTSA, the CO₂ is driven off from the adsorbent by heating the adsorbentin the regeneration phase. In PSA, the pressure of the purge gas islower than that of the feed gas and the change in pressure is used toremove the CO₂ from the adsorbent.

Other components can be removed from the feed air by these processes,including hydrocarbons and water. These adsorption techniques can alsobe applied to feed gases other than air or to air to be purified forpurposes other than use in an air separation plant. For example, otherapplications where trace or dilute CO₂ needs to be removed prior to acryogenic separation include the cryogenic production of CO fromsynthesis gas, the production of liquefied natural gas, and productionof dry CO₂-free air for a variety of applications.

The use of PSA for removing CO₂ from air prior to cryogenic airseparation is described in numerous publications, e.g. U.S. Pat. No.4,249,915 and U.S. Pat. No. 4,477,264. Initially, the practice was touse a dual bed of alumina for water removal followed by a zeolite suchas 13× for CO₂ removal. More recently, all alumina PSA systems have beenproposed, as described in U.S. Pat. No. 5,232,474. The advantages of anall alumina system include lower adsorbent cost, vessel design whichdoes not need screens to separate the two different adsorbents, andbetter thermal stability in the adsorption vessel during blowdown andrepressurization. It would be desirable however, to develop adsorbentshaving an improved CO₂ capacity so as to allow smaller bed sizes withlower capital costs and less void gas being lost during depressurizationi.e. higher air recoveries.

Alumina is also used as an adsorbent in TSA and for this purpose it hasbeen proposed to treat the alumina to form alkali metal oxides thereonto increase the adsorptive capacity of the alumina. By way of example,U.S. Pat. No. 4,493,715 teaches a method for removing CO₂ from olefinstreams by contacting the feed gas with a regenerable, calcinedadsorbent consisting essentially from 1 to 6 wt % of an alkali metaloxide selected from the group consisting of sodium, potassium, andlithium on alumina. The adsorbent was prepared by contacting aluminawith an alkali metal compound which is convertible to the metal oxide oncalcination.

U.S. Pat. No. 4,433,981 describes a process for removing CO₂ from agaseous stream which comprises contacting the gas steam at a temperatureup to about 300° C. with an adsorbent prepared by impregnation of aporous alumina with a sodium or potassium oxide. The corresponding oxidecan be prepared by impregnation with a decomposable salt and calciningat a temperature of 350° C. to 850° C. Salts mentioned include alkalimetal bicarbonates.

U.S. Pat. No. 3,557,025 teaches a method to produce alkalized aluminacapable of adsorbing SO₂ by selectively calcining the alumina, andcontacting with an alkali or ammonium bicarbonate salt to form at least30% by weight alkalized alumina having the empirical formulaMAl(OH)₂CO₃.

U.S. Pat. No. 3,865,924 describes the use of a finely ground mixture ofpotassium carbonate and alumina as an absorbent for carbon dioxide,which reacts with the carbonate and water to form bicarbonate. Theabsorbent mixture is regenerated by mild heating, e.g. at 93° C. (200°F.). The presence of stoichiometric quantities of water is essential andthe alumina appears to be regarded as essentially a mere carrier for thepotassium carbonate. Other carbonates may be used.

U.S. Pat. No. 5,232,474 discloses a PSA process using alumina in 70-100%of the bed volume to remove water and carbon dioxide from air.Preference is expressed for alumina containing up to 10 wt.% silica asopposed to the generality of aluminas which typically contain only about1% silica.

U.S. Pat. No. 5,656,064 discloses treatment of alumina with a basewithout calcining to form an alkali metal oxide to increasesubstantially the CO₂ adsorption capacity of the alumina that is capableof regeneration under PSA conditions.

U.S. Pat. No. 6,125,655 discloses a TSA process for purifying an airflow containing carbon dioxide and water vapor, in which at least someof the CO₂ and water vapor impurities are removed by adsorbing theimpurities on at least one calcined alumina containing at most 10% byweight (preferably 4 to 8 wt.%) of at least one alkali or alkaline-earthmetal oxide, the adsorption being carried out at a temperature ofbetween −10° C. and 80° C. For PSA cycles, no more than 5 wt.% alkali oralkaline earth metal promotions was preferred.

U.S. Pat. No. 7,759,288 discloses base treated aluminas that exhibitimproved CO₂ capacity compared to untreated aluminas. The base treatedaluminas prepared by physically mixing alumina and base during formingreportedly have (1) a higher surface area, (2) less hydrothermal aging,(3) improved CO₂ capacity, and (4) lower cost than base treated aluminasproduced by aqueous impregnation.

U.S. Pat. No. 5,656,064 and U.S. Pat. No. 6,125,655 teach that aphysical incorporation (incipient wetness impregnation on formed beadsor co-formed during bead rolling) of alkali carbonates or oxides onactivated alumina enhances CO₂ capacity, and is optimized by alkalitype, weight loading, and/or surface pH. The method of alkaliincorporation can provide differences as well, as U.S. Pat. No.7,759,288 teaches an improvement in capacity, surface area, and agingstability by co-forming instead of impregnation. The problem with thesecompositions, in PSA operation, is the loss in capacity between thefirst and second cycle of CO₂ exposure and regeneration purge. As U.S.Pat. No. 5,656,064 shows, CO₂ Henry's constants show at best 28% oftheir original CO₂ capacity after just one regeneration under vacuum atambient temperature.

Activated alumina adsorbents, promoted with alkali (e.g. Na, and K)compounds, are known for removing CO₂ from gas mixtures such as air.Alkali compounds increase the basicity of the alumina surface, whichincreases its affinity for CO₂ sorption. It is well-known in the artthat CO₂ will react with the alkali oxides to form alkali carbonates,and further reaction of CO₂ can occur in the presence of water vapor toform alkali bicarbonate.

Industry desires improved adsorbents for removing CO₂ from gas mixturescontaining low concentrations of CO_(2.)

Industry desires improved adsorbents for capturing CO₂ from ambient air.

Industry desires an improved process to produce adsorbents for removingCO₂ from gas mixtures.

Industry desires an improved device to produce dry, CO₂-free air.

Industry desires an improved device to produce dry, CO₂-free synthesisgas.

Industry desires an improved device to produce dry, CO₂-free naturalgas.

BRIEF SUMMARY

The present disclosure relates to an alkali-promoted activated aluminaadsorbent for removing CO₂ from a gas mixture containing CO₂, a methodof making the adsorbent, and a process using the adsorbent.

There are several aspects of the disclosure as outlined below. In thefollowing, specific aspects are outlined below. The reference numbersand expressions set in parentheses are referring to an exampleembodiment explained further below with reference to the figures. Thereference numbers and expressions are, however, only illustrative and donot limit the aspect to any specific component or feature of the exampleembodiment. The aspects can be formulated as claims in which thereference numbers and expressions set in parentheses are omitted orreplaced by others as appropriate.

Aspect 1. An adsorbent for use in a process to remove CO₂ from a gasmixture containing CO₂, the adsorbent comprising:

-   -   alumina;    -   a carbonate compound; and    -   one or more alkali metals wherein the adsorbent is 0.5 weight %        to 10 weight % or 1 to 10 weight % of the one or more alkali        metals;    -   wherein the surface area of the adsorbent ranges from 220 m²/g        to 400 m²/g; and    -   wherein the adsorbent has a carbonate to alumina intensity        ratio, R, the carbonate to alumina intensity ratio, R, having a        value less than or equal to 0.0150, where the carbonate to        alumina intensity ratio is as determined by Fourier Transform        infrared (FTIR) spectroscopy of a crushed sample of the        adsorbent, wherein the carbonate to alumina intensity ratio is a        ratio of a peak absorbance intensity for carbonate,        AI_(carbonate), to a peak absorbance intensity for alumina,        AI_(alumina), (i.e. R=AI_(carbonate)/AI_(alumina)), each peak        absorbance intensity obtained after subtracting a baseline        signal intensity, where the peak absorbance intensity for        alumina, AI_(alumina), is observed at an FTIR wavenumber in a        range from 420 cm⁻¹ to 520 cm⁻¹, and the peak absorbance        intensity for carbonate, AI_(carbonate), is observed at an FTIR        wavenumber in a range from 1300 cm⁻¹ to 1400 cm⁻¹.

Aspect 2. The adsorbent according to aspect 1 wherein the adsorbent is90 to 99 weight % alumina.

Aspect 3. The adsorbent according to aspect 1 or aspect 2 wherein thecarbonate compound is an alkali carbonate.

Aspect 4. The adsorbent according to aspect 3 wherein the alkalicarbonate is K₂CO₃.

Aspect 5. The adsorbent according to any one of aspects 1 to 4 whereinthe value of the carbonate to alumina intensity ratio ranges from 0.003to 0.0150 or ranges from 0.0035 to 0.013.

Aspect 6. A method for making adsorbent particles for adsorbing CO₂, theadsorbent particles comprising alumina, one or more alkali metals, and acarbonate compound, the method comprising:

-   -   washing alkali-promoted activated alumina materials comprising a        carbonate compound with water; and    -   drying the washed alumina materials to form the adsorbent        particles;    -   wherein the washed and dried adsorbent particles have a        carbonate to alumina intensity ratio, R₂, which is smaller than        the carbonate to alumina intensity ratio, R₁, of the        alkali-promoted activated alumina materials before washing;    -   wherein the carbonate to alumina intensity ratio is as        determined by Fourier Transform infrared (FTIR) spectroscopy of        a crushed sample of the respective washed and dried adsorbent        particles and a crushed sample of the alkali-promoted activated        alumina materials before washing, wherein the carbonate to        alumina intensity ratio is a ratio of a peak absorbance        intensity for carbonate, AI_(carbonate), to a peak absorbance        intensity for alumina, AI_(alumina), (i.e.        R=AI_(carbonate)/AI_(alumina)), each peak absorbance intensity        obtained after subtracting a baseline signal intensity, where        the peak absorbance intensity for alumina, AI_(alumina), is        observed at an FTIR wavenumber in a range from 420 cm⁻¹ to 520        cm⁻¹, and the peak absorbance intensity for carbonate,        AI_(carbonate), is observed at an FTIR wavenumber in a range        from 1300 cm⁻¹ to 1400 cm⁻¹.

Aspect 7. The method according to aspect 6 wherein the alkali-promotedactivated alumina materials are washed sufficiently to produce adsorbentparticles having a carbonate to alumina intensity ratio, R₁, having avalue less than or equal to 0.0150.

Aspect 8. The method according to claim 6 wherein the alkali-promotedactivated alumina materials are washed with water until thealkali-promoted activated alumina materials have a pH in solution of 9.5or less than 9.5 or less than 9 thereby forming washed aluminamaterials, the pH in solution as determined by measuring the pH of anequilibrated 2 liter solution of deionized water containing 100 g of thewashed alumina materials.

Aspect 9. A method for making adsorbent particles for adsorbing CO₂, theadsorbent particles comprising alumina, one or more alkali metals, and acarbonate compound, the method comprising:

-   -   washing alkali-promoted activated alumina materials comprising a        carbonate compound with water; and    -   drying the washed alumina materials to form the adsorbent        particles;    -   wherein the alkali-promoted activated alumina materials are        washed sufficiently to produce adsorbent particles having a        carbonate to alumina intensity ratio, R, the carbonate to        alumina intensity ratio, R, having a value less than or equal to        0.0150, where the carbonate to alumina intensity ratio is as        determined by Fourier Transform infrared (FTIR) spectroscopy of        a crushed sample of the adsorbent particles, wherein the        carbonate to alumina intensity ratio is a ratio of a peak        absorbance intensity for carbonate, AI_(carbonate), to a peak        absorbance intensity for alumina, AI_(alumina), (i.e.        R=AI_(carbonate)/AI_(alumina)), each peak absorbance intensity        obtained after subtracting a baseline signal intensity, where        the peak absorbance intensity for alumina, AI_(alumina), is        observed at an FTIR wavenumber in a range from 420 cm⁻¹ to 520        cm⁻¹, and the peak absorbance intensity for carbonate,        AI_(carbonate), is observed at an FTIR wavenumber in a range        from 1300 cm⁻¹ to 1400 cm⁻¹.

Aspect 10. A method for making adsorbent particles for adsorbing CO₂,the adsorbent particles comprising alumina, one or more alkali metals,and a carbonate compound, the method comprising:

-   -   washing alkali-promoted activated alumina materials comprising a        carbonate compound with water until the alkali-promoted        activated alumina materials have a pH in solution of 9.5 or less        than 9.5 or less than 9 thereby forming washed alumina        materials, where the pH in solution is as determined by        measuring the pH of an equilibrated 2 liter solution of        deionized water containing 100 g of the washed alumina        materials; and    -   drying the washed alumina materials to form the adsorbent        particles.

Aspect 11. The method according to any one of aspects 6 to 10 whereinthe alkali-promoted activated alumina materials have a surface arearanging from 220 m²/g to 400 m²/g.

Aspect 12. The method according to any one of aspects 6 to 11 whereinthe alkali-promoted activated alumina materials are washed with watercontaining less than 100 ppm total dissolved solids.

Aspect 13. The method according to any one of aspects 6 to 12 whereinthe alkali-promoted activated alumina materials are washed withdeionized water.

Aspect 14. The method according to any one of aspects 6 to 13 whereinthe washed alumina materials are dried in air, a vacuum, or anatmosphere containing greater than 79 mole % N₂.

Aspect 15. The method according to any one of aspects 6 to 14 whereinthe washed alumina materials are dried in an oven at a temperatureranging from 25° C. to 100° C.

Aspect 16. Adsorbent particles for use in a process to remove CO₂ from agas mixture containing CO₂ made in a method according to any one ofaspects 6 to 15.

Aspect 17. A process for removing CO₂ from a gas mixture containing CO₂,the process comprising:

-   -   passing the gas mixture containing CO₂ into a bed containing the        adsorbent according to any one of aspects 1 to 5 or a bed        containing the adsorbent particles according to aspect 16; and    -   withdrawing a CO₂-depleted gas from the bed.

Aspect 18. The process according to aspect 17 wherein the process is apressure swing adsorption process.

Aspect 19. The process according to aspect 17 wherein the process is atemperature swing adsorption process.

Aspect 20. The process according to any one of aspects 17 to 19 whereinthe gas mixture containing CO₂ has a concentration of CO₂ that is 1 mole% CO₂ or less than 1 mole % CO_(2.)

Aspect 21. The process according to any one of aspects 17 to 20 whereinthe gas mixture containing CO₂ has a concentration of CO₂ that is 5 ppmvor greater.

Aspect 22. The process according to any one of aspects 17 to 21 whereinthe gas mixture contains O₂, N₂, and H₂O.

Aspect 23. The process according to any one of aspects 17 to 21 whereinthe gas mixture contains CO, H₂, and H₂O.

Aspect 24. The process according to any one of aspects 17 to 21 whereinthe gas mixture contains CH₄, and H₂O.

Aspect 25. The process according to any one of aspects 17 to 22 whereinthe gas mixture is a feed to a cryogenic air separation unit.

Aspect 26. An adsorption unit comprising a bed containing the adsorbentaccording to any one of aspects 1 to 5 or the particles according toaspect 16.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a plot of end of feed CO₂ concentration versus feed contacttime for as-received (unwashed) and washed adsorbent AA4.

FIG. 2 is a plot of FTIR absorbance spectra for as-received (unwashed)and washed AA1 adsorbent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The ensuing detailed description provides preferred exemplaryembodiments only, and is not intended to limit the scope, applicability,or configuration of the disclosure. Rather, the ensuing detaileddescription of the preferred exemplary embodiments will provide thoseskilled in the art with an enabling description for implementing thepreferred exemplary embodiments, it being understood that variouschanges may be made in the function and arrangement of elements withoutdeparting from the scope as defined by the claims.

The articles “a” and “an” as used herein mean one or more when appliedto any feature in embodiments described in the specification and claims.The use of “a” and “an” does not limit the meaning to a single featureunless such a limit is specifically stated. The article “the” precedingsingular or plural nouns or noun phrases denotes a particular specifiedfeature or particular specified features and may have a singular orplural connotation depending upon the context in which it is used.

The adjective “any” means one, some, or all indiscriminately of whateverquantity.

The term “and/or” placed between a first entity and a second entityincludes any of the meanings of (1) only the first entity, (2) only thesecond entity, and (3) the first entity and the second entity. The term“and/or” placed between the last two entities of a list of 3 or moreentities means at least one of the entities in the list including anyspecific combination of entities in this list. For example, “A, B and/orC” has the same meaning as “A and/or B and/or C” and comprises thefollowing combinations of A, B and C: (1) only A, (2) only B, (3) onlyC, (4) A and B and not C, (5) A and C and not B, (6) B and C and not A,and (7) A and B and C.

The phrase “at least one of” preceding a list of features or entitiesmeans one or more of the features or entities in the list of entities,but not necessarily including at least one of each and every entityspecifically listed within the list of entities and not excluding anycombinations of entities in the list of entities. For example, “at leastone of A, B, or C” (or equivalently “at least one of A, B, and C” orequivalently “at least one of A, B, and/or C”) has the same meaning as“A and/or B and/or C” and comprises the following combinations of A, Band C: (1) only A, (2) only B, (3) only C, (4) A and B and not C, (5) Aand C and not B, (6) B and C and not A, and (7) A and B and C.

The term “depleted” means having a lesser mole % concentration of theindicated component than the original stream from which it was formed.“Depleted” does not mean that the stream is completely lacking theindicated component.

The terms “rich” or “enriched” means having a greater mole %concentration of the indicated component than the original stream fromwhich it was formed.

The present disclosure relates to an adsorbent comprising alumina, oneor more alkali metals, and a carbonate compound for use in a process toremove CO₂ from a gas mixture containing CO₂, a process and adsorptionunit for removing CO₂ from a gas mixture containing CO₂ using adsorbentparticles, and a method for making the adsorbent particles.

The adsorbent may be in any known particle form, for example, pellets,beads, powder, monoliths, laminates, or any other form known in the art.

The gas mixture may contain oxygen and nitrogen and may be a feed to acryogenic air separation unit. The gas mixture may have a concentrationof CO₂ that is 1 mole % or less than 1 mole %. The gas mixture may havea concentration of CO₂ that is greater than 5 ppmv CO_(2.) The gasmixture may also contain water and the adsorbent may also remove waterfrom the gas mixture. The adsorbent may demonstrate simultaneous waterand CO₂ removal.

The adsorbent according to the present disclosure comprises alumina, acarbonate compound, and one or more alkali metals. The adsorbent maycontain 0.5 weight % to 10 weight % or 1 weight % to 10 weight % of theone or more alkali metals. The weight % of the one or more alkali metalsis the weight % of the respective weight of the alkali metal, not theweight % of the alkali compound. The one or more alkali metals arepresent in ionic form. The adsorbent may be 90 to 99 weight % alumina.The carbonate compound may be an alkali carbonate, for example, K₂CO₃.The weight % alkali metal may be determined by X-ray fluorescence (XRF)spectroscopy.

The alumina and carbonate compound may be co-formed or spray-formed toform the adsorbent.

The surface area of the adsorbent ranges from 220 m²/g to 400 m²/g.Adsorbents with surface areas in this range are suitable for airpre-purification. Alumina adsorbents used in air pre-purificationtypically have a surface area greater than 220 m²/g because this surfacearea is required to maintain high water capacity, and the aluminaadsorbent must also be capable of removing water in addition to CO₂ toprevent solidification of both CO₂ and water in downstream cryogenicprocesses. Adsorbents having a surface area less than 220 m²/g aregenerally not suitable for removing water from the feed gas mixture forair pre-purification for a cryogenic air separation plant.

The present inventors have discovered that washing an alkali saltpromoted activated alumina adsorbent with water significantly improvesthe performance of the adsorbent for removing CO₂ from air in pressureswing and temperature swing adsorption processes. The adsorbent retainssome alkali after washing so that enhanced CO₂ capacity remains, butwater soluble carbonate species that appear to hinder the adsorbent'scyclic ability to sorb and desorb CO₂ are removed from the adsorbent.

The adsorbent may be characterized by a carbonate to alumina intensityratio, R, having a value less than or equal to 0.0150 as determined byFourier Transform infrared (FTIR) spectroscopy of a crushed sample ofthe adsorbent. The value of the carbonate to alumina intensity ratio mayrange from 0.003 to 0.0150 or may range from 0.0035 to 0.013.

This carbonate to alumina intensity ratio is lower than the carbonate toalumina intensity ratios found in prior art adsorbents.

The carbonate to alumina intensity ratio correlates to the ratio of themass fraction of carbonate to alumina.

The Fourier Transform infrared (FTIR) spectroscopy may be done, forexample, using a Nicolet Nexus 670 FTIR interferometer.

The crushed sample is formed by crushing a sample of the adsorbent, forexample, using a mortar and pestle. The crushed sample may have a meanparticle size ranging from 10 microns to 300 microns. The particle sizemay be determined by the method described by Eshel et al. in Soil Sci.Soc. Am. J. 68:736-743 (2004), using a Horiba LA-950 laser particle sizeanalyzer.

As part of the measurement method, the crushed sample of adsorbent ispressed on a diamond crystal in a SmartORBIT™ Attenuated TotalReflectance accessory.

The carbonate to alumina intensity ratio is a ratio of a peak absorbanceintensity for carbonate, AI_(carbonate), to a peak absorbance intensityfor alumina, AI_(alumina), (i.e. R=AI_(carbonate)/AI_(alumina)), eachpeak absorbance intensity obtained after subtracting a baseline signalintensity. The peak absorbance intensity for alumina and the peakabsorbance intensity for carbonate are observed by Fourier Transforminfrared (FTIR) spectroscopy of the crushed sample of the adsorbent. Thepeak absorbance intensity for alumina, AI_(alumina), is observed at anFTIR wavenumber in a range from 420 cm⁻¹ to 520 cm⁻¹, and the peakabsorbance intensity for carbonate, AI_(carbonate), is observed at anFTIR wavenumber in a range from 1300 cm⁻¹ to 1400 cm⁻¹. The peakabsorbance intensity is the maximum value in the specified range.

The baseline signal intensity is a linear baseline function determinedfrom two local minimum values of absorbance intensity betweenfrequencies ranging from 1100 cm⁻¹ to 1800 cm⁻¹. Determining baselinesignal intensities and subtracting baseline signal intensities fromabsorbance intensity spectra is routine and standard in the field ofFTIR spectroscopy.

The peak absorbance intensity for alumina and the peak absorbanceintensity for carbonate may be as determined from an FTIR spectrum asobtained by co-adding multiple scans. In the present case, it has beenfound beneficial to co-add 128 scans at 4 cm⁻¹ resolution using aNicolet Nexus 670 FTIR interferometer.

The present disclosure also relates to methods for making adsorbentparticles comprising alumina, one or more alkali metals, and a carbonatecompound for adsorbing CO₂ having improved sorbing and desorbingcharacteristics.

The methods for making adsorbent particles comprise washingalkali-promoted activated alumina materials comprising a carbonatecompound with water, and drying the washed alumina materials to form theadsorbent particles. The materials are preferably washed with watercontaining less than 100 ppm total dissolved solids, and more preferablydeionized water. Total dissolved solids may be measured, for example, bythe method described in American Society of Testing and Materials (ASTM)D5907-13.

Alkali-promoted activated alumina materials comprising a carbonatecompound are available commercially, for example, from Axens, BASF,Porocel and/or UOP.

Alkali-promoted activated alumina materials may be formed byincorporating an alkali metal carbonate into activated alumina materialsto form alkali-promoted alumina materials. The alkali-promoted aluminamaterials may be calcined in an atmosphere, for example an airatmosphere, to form the alkali-promoted activated alumina materials,which are subsequently cooled. Calcining temperatures typically rangefrom about 300° C. to 350° C. Activated alumina production is well-knownand described, for example, in U.S. Pat. Nos. 3,226,191, 4,568,664, and5,935,894.

The alkali-promoted activated alumina materials may be made byincorporating alkali metal carbonate into an activated alumina structureby spray coating an alumina support structure. The alkali-promotedactivated alumina materials may be made by incorporating alkali metalcarbonate into an activated alumina structure by co-forming the alkalimetal carbonate with alumina to form the alkali-promoted activatedalumina materials. Any known process for making alkali-promotedactivated alumina materials may be used.

The washing may be done in a batch or continuous process.

Washing does not remove all of the alkali—some remains either as waterinsoluble K₂Al(CO₃)₂OH dawsonite phase, or potassium cationsintercalated into gibbsite phases on the activated alumina surface. Thisremaining alkali enhances the surface basicity enough to increaseequilibrium CO₂ capacity compared to unpromoted alumina, but does nothinder CO₂ kinetics. Pure K₂CO₃ can limit rates of CO₂ sorption by itsrelatively slow reaction with CO₂ as described by Rahimpour et al., inChemical Engineering and Processing 43 (2004) pp. 857-865.

The carbonate to alumina intensity ratio of a crushed sample of theadsorbent particles as determined by FTIR spectroscopy is decreased bywashing the adsorbent with water.

The alkali-promoted activated alumina materials may be washedsufficiently to produce adsorbent particles having a carbonate toalumina intensity ratio, R₂, which is smaller than the carbonate toalumina intensity ratio, R₁, of the alkali-promoted activated aluminamaterials before washing.

The alkali-promoted activated alumina materials may be washedsufficiently to produce adsorbent particles having a carbonate toalumina intensity ratio, R, having a value less than or equal to 0.0150as determined by Fourier Transform infrared (FTIR) spectroscopy of acrushed sample of the adsorbent particles. The value of the carbonate toalumina intensity ratio may range from 0.003 to 0.0150 or may range from0.0035 to 0.013 as a result of washing with water.

The description above relating to the adsorbent regarding the carbonateto alumina intensity ratio, FTIR spectroscopy, crushed sample, etc.applies also to this embodiment of the method for making adsorbentparticles.

The pH of a washing solution in equilibrium with a quantity of adsorbentafter washing is also decreased by washing the adsorbent with water. Themethod of making the adsorbent particles can therefore also becharacterized by the pH of a washing solution after washing theadsorbent.

The alkali-promoted activated alumina materials may be washed with wateruntil the alkali-promoted activated alumina materials have a pH insolution of 9.5 or less than 9.5 or less than 9 thereby forming washedalumina materials. The pH in solution is determined by measuring the pHof an equilibrated 2 liter solution of deionized water containing 100 gof the washed alumina materials. The solution may be consideredequilibrated if the pH does not change after repeated measurements overthe period of several hours. The pH may be measured using a digitalFisher Science Education pH/ion 510 meter. The pH meter may becalibrated with standard buffer solutions at 4.0, 7.0, and 10.0.

Alkali-promoted activated alumina materials as received from a supplierhave a pH in solution of greater than 9 where the pH in solution is asdetermined by measuring the pH of an equilibrated 2 liter solution ofdeionized water containing 100 g of the as-received alkali-promotedactivated alumina materials.

The washed alumina materials may be dried in an oven at a temperatureranging from 25° C. to 100° C.

The washed alumina materials may be dried in air, a vacuum, or anatmosphere containing greater than 79 mole % N₂ to 100 mole % N₂, or anatmosphere containing Ar or He. Generally, the drying atmosphere shouldpreferably contain less than 50 ppmv CO₂ and contain water with a dewpoint ranging from −90° C. to −40° C.

The present disclosure also relates to a process for removing CO₂ from agas mixture containing CO_(2.) The gas mixture may contain oxygen andnitrogen, and may be a feed to a cryogenic air separation unit. The gasmixture may have a concentration of CO₂ that is 1 mole % CO₂ or lessthan 1 mole % CO₂. The gas mixture may have a concentration of CO₂ thatis greater than 5 ppmv. The gas mixture may also contain water and theadsorbent may also remove water from the gas mixture.

The process comprises passing the gas mixture containing CO₂ into a bedcontaining the adsorbent having the desired characteristics as describedabove or made by the methods described above, and withdrawing aCO₂-depleted gas from the bed.

The process may be a pressure swing adsorption (PSA) process. Pressureswing adsorption is well-known. PSA cycles suitable for use with thepresent adsorbent include U.S. Pat. Nos. 5,656,065, 5,919,286,5,232,474, 4,512,780, 5,203,888, 6,454,838, and 5,156,657, and U.S. Pat.Appl. No. 2014/0373713.

The process may be a temperature swing adsorption (TSA) process.Temperature swing adsorption is well-known. TSA cycles suitable for usewith the present adsorbent include U.S. Pat. Nos. 5,614,000, 5,855,650,7,022,159, 5,846,295, 5,137,548, 4,541,851, 4,233,038, and 3,710,547.

The present disclosure also relates to an adsorption unit. Theadsorption unit comprises a bed containing the adsorbent having thedesired characteristics as described above and/or made by the methodsdescribed above.

EXAMPLES

Several samples of commercially-supplied activated alumina beads havinga bead diameter of 2 mm, were washed with deionized water. Thespecifications for each of the samples are shown in Table 1.

Adsorbent AA1 is promoted with 5 wt % potassium carbonate. The adsorbentis a spray-formed adsorbent, and falls within the scope of the adsorbentdescribed in U.S. Pat. No. 5,656,064. 102 grams of the adsorbent wasadded to 2 liters of deionized water and manually stirred for severalminutes. After equilibration, the pH of this 2 liter water solution was11. The pH was measured using a digital Fisher Science Education pH/ion510 meter, calibrated with standard buffer solutions at 4.0, 7.0, and10.0. The solution was decanted. This soaking/washing process wasrepeated 14 times, after which the beads were filtered from the washingsolution with a Buchner funnel. The solution over the alumina beads onthe final wash measured a pH of 9.

After filtration the beads were dried in air in an oven at 90° C.

Other adsorbents were similarly washed and dried. These adsorbentsincluded potassium carbonate co-formed adsorbent, AA2, which fallswithin the scope of the adsorbent described in U.S. Pat. No. 7,759,288,sodium oxide promoted activated alumina, AA3, which falls within thescope of U.S. Pat. No. 6,125,655, and AA4, another commerciallyavailable potassium carbonate co-formed adsorbent.

TABLE 1 Adsorbent AA1 AA2 AA3 AA4 Surface area 230 266 230 261 (m²/g)Bulk density 873 913 766 791 (kg/m³) Particle size 95% 95% 95% 95%distribution between between between between 8 × 14 8 × 14 8 × 14 8 × 14Tyler mesh Tyler mesh Tyler mesh Tyler mesh Alkali K₂CO₃ K₂CO₃ Na₂OK₂CO₃ promoter Promoter  5  8  13  8 loading (wt %)

Each of the adsorbents was tested in a pressure swing adsorption testrig. Unwashed and washed samples of the adsorbents were tested.

For each adsorbent test, a 1.9 cm (0.75 inch) diameter by 45.7 cm (18inch) tall vessel was filled with respective adsorbent particles. Thesingle vessel was cycled under PSA conditions with a feed gas flowing at5 liters per minute for 10 minutes at 308 kPa absolute (30 psig)pressure, followed by depressurization to 136 kPa absolute (5 psig), anda regeneration purge gas flowing at 3.5 liters per minute for 10 minutesat 136 kPa absolute (5 psig) pressure. The feed gas was a gas mixturecontaining air with 400 ppm CO_(2.) The purge gas was N₂. The vesselcontaining the respective adsorbents were cycled until a steady statewas achieved.

The concentration of CO₂ at the exit end of the vessel was measuredduring the feed step. The concentration of CO₂ at the end of the feedstep was recorded and a mean value of the CO₂ concentration at the endof feed step calculated for 10 cycles after steady state was achieved.The mean values of the CO₂ concentration at the end of feed for each ofthe adsorbents in as-received and washed are shown in Table 2.

TABLE 2 AA1 AA2 as-received washed as-received washed CO₂ (ppm) 58.034.7 72.8 36.0 AA3 AA4 as-received washed as-received washed CO₂(ppm) >100 16.3 >100 39.5

It is shown that water washed versions of promoted alumina adsorbentsreduce the amount of CO₂ present at the end of PSA feed steps in cyclicoperation, demonstrating that the washed alumina is removing more CO₂than the as-received promoted alumina under equivalent processconditions.

In another series of tests, adsorbent AA4 was cycled with incrementallyslower feed and purge gas flow rates, and longer feed and purge steptimes such that the total volume of feed gas (50 liters) and totalvolume of purge gas (35 liters) processed was the same as the PSA testconditions described above. The resulting effect was an increase to theempty bed contact time (gas flow rate divided by empty bed volume) inboth feed and regeneration steps. As shown in FIG. 1, the unwashed AA4requires a much longer contact time to achieve the same CO₂ removalperformance as washed AA4. The results show that washing the adsorbentsimproves the kinetics of CO₂ adsorption and desorption such thatequivalent PSA performance can be achieved with 45% less contact timevs. un-washed alumina adsorbent. The improved kinetics reduces bed sizeand associated capital costs.

The impact of washing on the percent alkali utilization was alsodetermined. The percent alkali utilization, or alkali utilization isdefined as:

100*(C _(PAA) −C _(AA))/C _(CO2)

where

-   C_(PAA)=CO₂ capacity of promoted activated alumina, in mmol/g;-   C_(AA)=CO₂ capacity of unpromoted activated alumina, in mmol/g; and-   C_(CO2)=calculated capacity if all K₂CO₃ loaded on alumina reacted    with CO₂, in mmol/g.

Unpromoted activated alumina, as-received (unwashed) and washed AA4 weretested using a thermogravimetric analyzer (TGA) and potassium contentmeasured by XRF). The as-received AA4 had 4.9 weight % K, correspondingto 0.00063 moles K₂CO₃/g, and the washed AA4 had 3.1 weight % K,corresponding to 0.00040 moles K₂CO₃/g.

For each of the samples, 50 mg of adsorbent were loaded into the TGA. Aninitial drying step under pure N₂ to 100° C. was performed. While heldisothermal at 30° C., 1% CO₂ in N₂ was flowed over the sample at 50mL/min for 60 min. The sweep gas was then switched to pure N₂,maintained at 30° C., and flowed for another 60 min. These latter twosteps were repeated 5 times, and the weight change between the beginningand end of the 1% CO₂ in N₂ step recorded as CO₂ uptake capacity.

The results of the TGA are shown in Table 3.

For as-received AA4 (K₂CO₃ promoted alumina), the utilization is shownto be about 40% in the first exposure to CO₂, and drops to only 4% after5 cycles of CO₂ exposure and ambient regeneration in N₂. To contrast, itis demonstrated that after water washing the AA4 adsorbent, the firstcycle utilization of K₂CO₃ improves to 50%, while cycle 5 utilizationimproves to 12%, which is 3 times that of the sample without washing. Inaddition, the washed sample displays a slower rate of capacity loss thanthe unwashed sample. The unwashed retains only 22% (0.080/0.362) of itsoriginal capacity on the 5^(th) cycle, while the washed sample retains32% (0.104/0.323) of its original capacity.

TABLE 3 CO₂ capacity K₂CO₃ Utilization (mmol/g) (%) Cycle 1 Cycle 5Cycle 1 Cycle 5 Unpromoted activated alumina 0.125 0.054 — — AA4 0.3620.080 38  4 Washed AA4 0.323 0.104 50 12

The change of the composition of each adsorbent as a result of washingeach adsorbent was determined using Fourier Transform infrared (FTIR)spectroscopy.

Each of the adsorbents, both as-received and washed samples, were eachindividually manually ground with a mortar and pestle until the meanparticle size was between 10 and 300 microns as determined using aHoriba LA-950 laser particle analyzer. The resulting powder was dried inan oven at 90° C. for 12 hours in an air atmosphere. After drying, eachpowder sample was pressed onto a diamond crystal in a SmartORBIT™ ATRaccessory and a spectrum was obtained by co-adding 128 scans at 4 cm⁻¹resolution with a Nicolet Nexus 670 FT-IR interferometer.

An adsorbance spectrum for AA1 is shown in FIG. 2. The peak absorbanceintensity for alumina is observed at an FTIR wavenumber in a range from420 cm⁻¹ to 520 cm⁻¹, and the peak absorbance intensity for carbonate isobserved at an FTIR wavenumber in a range from 1300 cm⁻¹ to 1400 cm⁻¹.

The measured carbonate and alumina peak intensities are corrected bysubtracting the baseline signal intensity, measured as a linear baselinefunction determined from two local minimum values of absorbanceintensity between frequencies ranging from 1100 cm⁻¹ to 1800 cm⁻¹. Forexample, the baseline function for the FTIR spectra measured for AA1,was calculated using values at wavelengths of 1292 cm⁻¹ and 1751 cm⁻¹,where the absorbance intensities (Al) were 0.0587 and 0.0574,respectively. The linear function:

AI=a*W+b

where

-   AI=absorbance intensity-   W=wavelength in cm⁻¹-   a=(0.0587−0.0574)/(1292−1751)=−2.83−10⁻⁶-   b=0.0587−(−2.83×10⁻⁶)*1292=0.0624-   was determined. The absorbance intensity of the baseline at the    carbonate peak wavenumber of 1359 cm-1, was then calculated as:

AI=−2.83×10−6*1359+0.0624=0.0585.

The same baseline absorbance intensity is used for the alumina peakwavenumber, due to the relative closeness in frequency of the aluminaand carbonate wavenumbers, and the insignificance of baseline variationversus the larger intensity of the alumina peak.

The measured intensities, baseline intensity, and baseline subtractedcorrected intensities taken from the adsorbance spectrum shown in FIG. 2are summarized in Table 4 for AA1 and washed AA1. The baseline correctedcarbonate peak intensity is the peak intensity minus the baselineintensity. For example, the baseline corrected carbonate peak intensityis 0.0724−0.0585=0.0139.

TABLE 4 Baseline Baseline corrected corrected Carbonate Aluminacarbonate alumina peak peak Baseline peak peak intensity intensityintensity intensity intensity AA1 as 0.0724 0.930 0.0585 0.0139 0.872received AA1 0.0638 1.02 0.0604 0.00340 0.955 washed

The baseline corrected absorbance intensity for the carbonate peak andthe alumina peak, along with the carbonate to alumina intensity ratio,are shown in Table 5 for each of the adsorbents, as-received and washed.The carbonate to alumina intensity ratio is calculated by dividing thebaseline corrected carbonate peak intensity by the baseline correctedalumina peak intensity, e.g. for the as-received AA1, the carbonate toalumina intensity ratio is 0.0139/0.872=0.0159.

TABLE 5 AA1 AA2 absorbance intensity (counts/sec) as-received washedas-received washed carbonate 0.0139 0.00340 0.0151 0.00970 alumina 0.8720.955 0.700 0.713 carbonate/alumina 0.0159 0.00356 0.0216 0.0136 AA3 AA4absorbance intensity (counts/sec) as-received washed as-received washedcarbonate 0.0267 0.0122 0.0121 0.00860 alumina 0.730 0.985 0.666 0.769carbonate/alumina 0.0366 0.0124 0.0182 0.0112

The results in Table 5 show that that the carbonate to alumina intensityratio is significantly reduced as a result of washing the adsorbent,which correspondingly shows that the carbonate species are significantlyreduced as a result of washing the adsorbent.

As a result of washing the adsorbents the carbonate to alumina intensityratio is reduced below 0.0150 for all of the adsorbents, a value lessthan any of the as-received adsorbents.

It is unexpected that washing does not remove the majority of thepotassium given the high water solubility of K₂CO₃. For the AA1adsorbent, potassium content only drops from 4.9 wt % to 3.1 wt % afterwashing (63% remains on the surface). Table 1 in U.S. Pat. No. 7,759,288patent shows CO₂ capacity increases as you go from 0, 5, and 8 wt %K₂CO₃. The result in this disclosure, where removing carbonate from thealumina increases CO₂ capacity in cyclic operation is highly unexpected.

In view of the results shown in Table 2 above, it is seen that the lowcarbonate containing compositions allow higher utilization of thealkali, particularly for PSA cycles and low temperature TSA cycles.

In this disclosure, it is shown that by removing the unreacted alkalicarbonates via water washing, the kinetics of CO₂ sorption/desorption isimproved, cyclic CO₂ capacity is increased, and utilization of the K₂CO₃is increased. This leads to better overall performance in PSA cyclicconditions compared to as-received promoted activated alumina as shownin Table 2.

The utility the present adsorbent provides is a new composition ofalumina that provides better CO₂ removal performance in a PSA system,particularly for CO₂ removal prior to cryogenic distillation of air, anda method of manufacture where alkali carbonate or oxide promotedactivated alumina can be modified, regardless of promotion method, forimproved use in PSA cycles to remove CO₂ from a gas composition.

The washing step is shown to improve PSA performance in various promotedaluminas, including salt sprayed, co-formed, and Na or K impregnatedspecies. IR spectroscopy confirms that alkali promoted aluminas can bemodified to a unique composition after washing, where very littlecarbonate remains in the material.

1. An adsorbent for use in a process to remove CO₂ from a gas mixturecontaining CO₂, the adsorbent comprising: alumina; a carbonate compound;one or more alkali metals wherein the adsorbent is 0.5 to 10 weight % ofthe one or more alkali metals; wherein the surface area of the adsorbentranges from 220 m²/g to 400 m²/g; and wherein the adsorbent has acarbonate to alumina intensity ratio, R, the carbonate to aluminaintensity ratio, R, having a value less than or equal to 0.0150, wherethe carbonate to alumina intensity ratio is as determined by FourierTransform infrared (FTIR) spectroscopy of a crushed sample of theadsorbent, wherein the carbonate to alumina intensity ratio is a ratioof a peak absorbance intensity for carbonate, AI_(carbonate), to a peakabsorbance intensity for alumina, AI_(alumina), each peak absorbanceintensity obtained after subtracting a baseline signal intensity,-wherethe peak absorbance intensity for alumina, AI_(alumina), is observed ata wavenumber in a range from 420 cm⁻¹ to 520 cm⁻¹, and the peakabsorbance intensity for carbonate, AI_(carbonate), is observed at awavenumber in a range from 1300 cm⁻¹ to 1400 cm⁻¹.
 2. The adsorbentaccording to claim 1 wherein the adsorbent is 90 to 99 weight % alumina.3. The adsorbent according to claim 1 wherein the carbonate compound isan alkali carbonate.
 4. The adsorbent according to claim 3 wherein thealkali carbonate is K₂CO₃.
 5. The adsorbent according to claim 1 whereinthe value of the carbonate to alumina intensity ratio ranges from 0.003to 0.0150.
 6. A method for making adsorbent particles for adsorbing CO₂,the adsorbent particles comprising alumina, one or more alkali metals,and a carbonate compound, the method comprising: washing alkali-promotedactivated alumina materials comprising a carbonate compound with water;and drying the washed alumina materials to form the adsorbent particles;wherein the washed and dried adsorbent particles have a carbonate toalumina intensity ratio, R₂, which is smaller than the carbonate toalumina intensity ratio, R₁, of the alkali-promoted activated aluminamaterials before washing; wherein the carbonate to alumina intensityratio is as determined by Fourier Transform infrared spectroscopy of acrushed sample of the respective washed and dried adsorbent particlesand a crushed sample of the alkali-promoted activated alumina materialsbefore washing, wherein the carbonate to alumina intensity ratio is aratio of a peak absorbance intensity for carbonate, AI_(carbonate), to apeak absorbance intensity for alumina, AI_(alumina), (i.e.R=AL_(carbonate)/AI_(alumina)) each absorbance intensity obtained aftersubtracting a baseline signal intensity, where the peak absorbanceintensity for alumina, AI_(alumina), is observed at an FTIR wavenumberin a range from 420 cm⁻¹ to 520 cm⁻¹, and the peak absorbance intensityfor carbonate, AI_(carbonate), is observed at an FTIR wavenumber in arange from 1300 cm⁻¹ to 1400 cm⁻¹.
 7. The method according to claim 6wherein the water is deionized water.
 8. The method according to claim 6wherein the alkali-promoted activated alumina materials are washedsufficiently to produce adsorbent particles having a carbonate toalumina intensity ratio, R₁, having a value less than or equal to0.0150.
 9. The method according to claim 6 wherein the alkali-promotedactivated alumina materials are washed with water until thealkali-promoted activated alumina materials have a pH in solution of 9.5or less thereby forming washed alumina materials, the pH in solution asdetermined by measuring the pH of an equilibrated 2 liter solution ofdeionized water containing 100 g of the washed alumina materials.
 10. Amethod for making adsorbent particles for adsorbing CO₂, the adsorbentparticles comprising alumina, one or more alkali metals, and a carbonatecompound, the method comprising: washing alkali-promoted activatedalumina materials comprising a carbonate compound with water; and dryingthe washed alumina materials to form the adsorbent particles; whereinthe alkali-promoted activated alumina materials are washed sufficientlyto produce adsorbent particles having a carbonate to alumina intensityratio, R, the carbonate to alumina intensity ratio, R, having a valueless than or equal to 0.0150, where the carbonate to alumina intensityratio is as determined by Fourier Transform infrared spectroscopy of acrushed sample of the adsorbent particles, wherein the carbonate toalumina intensity ratio is a ratio of a peak absorbance intensity forcarbonate, AI_(carbonate), to a peak absorbance intensity for alumina,AI_(alumina), (i.e. R=AI_(carbonate)/AI_(alumina)), each peak absorbanceintensity obtained after subtracting a baseline signal intensity, wherethe peak absorbance intensity for alumina, AI_(alumina), is observed atan FTIR wavenumber in a range from 420 cm⁻¹ to 520 cm⁻¹, and the peakabsorbance intensity for carbonate, AI_(carbonate), is observed at anFTIR wavenumber in a range from 1300 cm⁻¹ to 1400 cm⁻¹.
 11. A method formaking adsorbent particles for adsorbing CO₂, the adsorbent particlescomprising alumina, one or more alkali metals, and a carbonate compound,the method comprising: washing alkali-promoted activated aluminamaterials comprising a carbonate compound with water until thealkali-promoted activated alumina materials have a pH in solution of 9.5or less thereby forming washed alumina materials, where the pH insolution is as determined by measuring the pH of an equilibrated 2 litersolution of deionized water containing 100 g of the washed aluminamaterials; and drying the washed alumina materials to form the adsorbentparticles.
 12. Adsorbent particles for use in a process to remove CO₂from a gas mixture containing CO₂ made in a method according to claim 6.13. Adsorbent particles for use in a process to remove CO₂ from a gasmixture containing CO₂ made in a method according to claim
 10. 14.Adsorbent particles for use in a process to remove CO₂ from a gasmixture containing CO₂ made in a method according to claim
 11. 15. Aprocess for removing CO₂ from a gas mixture containing CO₂, the processcomprising: passing the gas mixture containing CO₂ into a bed containingthe adsorbent according to claim 1; and withdrawing a CO₂-depleted gasfrom the bed.
 16. The process according to claim 15 wherein the gasmixture containing CO₂ has a concentration of CO₂ that ranges from 5ppmv CO₂ to 1 mole % CO_(2.)
 17. The process according to claim 15wherein the gas mixture contains oxygen, nitrogen, and water.
 18. Theprocess according to claim 15 wherein the gas mixture is a feed to acryogenic air separation unit.
 19. An adsorption unit comprising a bedcontaining the adsorbent according to claim
 1. 20. An adsorption unitcomprising a bed containing the adsorbent particles according to claim12.